Toner pickoff apparatus

ABSTRACT

Weakly charged toner particles are removed from a developer mix prior to developing an electrostatic latent image by bringing the developer in contact with a biased roller. Also, toner that is charged to the wrong polarity may be removed from the developer mix by a biased roller prior to developing the electrostatic latent image. The removal of the toner particles from the developer prior to presenting the developer to an electrostatic latent image substantially reduces background when developing the latent image.

Oct. 14, 1975 United States Patent [191.

Volkers Hudson. 118/637 Berlier et a1. 117/17 5 1/1974 Urbanek et a1.

3,662,71 l 5/1972 3,754,962 8/1973 3,783,826 118/620 3,818,864 6/1974Bickmore............................ 118/637 1 TONER PICKOFF APPARATUS[75] lnventor: Stewart William Volkers,

Williamson, NY.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

[22] Filed: Mar. 30, 1973 [21] Appl. No.: 346,485

developer mix prior to developing an electrostatic 1atent image bybringing the developer in contact with a biased roller. Also, toner thatis charged to the wrong 12 Claims, 2 Drawing Figures polarity may beremoved from the developer mix by a biased roller prior to developingthe electrostatic latent image. The removal of the toner particles fromthe developer prior to presenting the developer to an 1 37 electrostaticlatent image substantially reduces background when developing the latentimage.

References Cited UNITED STATES PATENTS US. Patent Oct. 14, 1975 a 7 IIII .I I I I I I --anavp conapn--anaan TONER PICKOFF APPARATUS DESCRIPTIONOF THE INVENTION In developing electrostatic latent images, a developermaterial. comprising toner particles and carrier particles. is presentedto the electrostatic latent image for developing the same. The tonerparticles are triboelec trically charged through a mixing and tumblingaction with the carrier particles present in the developer mix.Normally, however there are some toner particles which carry a lowcharge and some toner particles which are charged to the wrong polarity.The quantity of improperly charged toner particles increases with theage ofthe developer due to carrier impaction. It has been found that thebackground development level is a function of the concentration ofimproperly charged toner particles in the developer with backgroundbecoming worse with an increase in this type of toner. The reasons arenot fully understood, but some general observations may be made. Whenpositive-to-positive reproduction is desired, and the electrostaticlatent image carries a positive charge, then the carrier will have apositive charge and the toner a negative charge. Normally, theelectrostatic latent image may have a charge of +800 volts and thebackground will have a charge of around +200 volts and the toner willcarry negative charge.

One explanation of the above phenomenon is that assuming theelectrostatic latent image carries a positive charge of +800 volts andthe background a charge of +200 volts, the toner particles of lownegative charge will be attracted by both the background charge and theimage charge while the more negatively charged toner will bepreferentially strongly attracted by the high charge on the image. It isalso felt that toner particles of low charge and charged to the wrongpolarity are mechanically deposited on the background. When adevelopment electrode is utilized, development of background areas issuppressed whereby toner charged to the correct polarity is repelledfrom the background areas, but the toner charged to the wrong polarityis developed on the background.

It is therefore an object of this invention to eliminate the abovedisadvantages and thereby suppress background development by withdrawinglow or weakly charged toner particles from the developer prior topresenting the developer to an electrostatic latent image.

lt is a further object of this invention to eliminate the abovedisadvantages and thereby suppress background by withdrawing tonerparticles, which are charged to the wrong polarity, from the developerprior to presenting the developer to an electrostatic latent image.

A still further object of this invention is to suppress development ofselected lower contrast electrostatic images by removing lower chargedtoner particles from the developer prior to presenting the developer toan electrostatic image. i

Other objects of the invention will become apparent from the followingdescription with reference to the drawings wherein:

FIG. 1 is a view of the development system utilizing the principles ofthis invention; and

FIG. 2 is a view ofa magnetic brush development system utilizing theprinciples of this invention.

Referring to FIG. 1, a drum 10 has a photosensitive surface 12 thereonbearing an electrostatic latent image. The drum 10 is rotated in aclockwise direction by a motor 14 outlined in block form. A developerhousing 16 is located adjacent the drum and contains a toner reservoir18 having a supply of toner 20 therein and a developer sump having asupply of developer 22 therein. The developer comprises carrier andtoner particles so selected that a triboelectric attraction existsbetween them causing the two particles to cling together and acquire anopposite charge. Each carrier has numerous toner particles attractedthereon allowing them to be transferred into contact with thephotosensitive surface where the greater electrostatic attraction of thelatent image will overcome the triboelectric attraction between the twodeveloper components causing toner to be stripped off the carrier andelectrostatically bonded to the charged image to effect developmentthereof. A bucket conveyor 24 is rotatably mounted in the housing 16 andis arranged to convey developer 22 from the developer sump upwards anddeposit the same onto a chute 26. A motor 28 shown in block diagram, ispositively connected to a pulley 30 to rotate the pulley and thereby thebucket conveyor 24. A toner pickoff rotatable biased roller 32 islocated at the bottom of the chute 26 and has a motor 34 (shown in blockdiagram) positively connected thereto for rotating the same in aclockwise direction. The roller 32 collects toner particles thereon in amanner to be described hereafter. A biased development electrode 33 isprovided adjacent the roller 32 to develop toner particles onto theroller 32. A combined scraper and guide member 36 is located in thehousing and is tangentially disposed relative to the surface of theroller 32 to engage and scrape off any toner particles collected on theroller 32 and guide the same back into the developer sump withoutengaging the photosensitive surface.

Assuming an image of a positive charge, when it is desired to withdrawweakly charged negative toner, the roller 32 has a positive voltage Vapplied thereto and the development electrode 33 has either a negativeor less positive voltage V applied thereto. The difference in thevoltage potentials V, and V is usually such as to produce an attractivefield which is equal to or slightly greater than the attractive field inthe background areas of the latent image.

When developer passes over the rotating roller 32, the attractive fieldbetween the roller 32 and electrode 33 will be insufficient to striptoner from its corresponding carrier if the triboelectric attractiontherebetween is greater than the field due to the potential between theroller 32 and electrode 33. However, if the triboelectric attractionbetween the toner and its associated carrier is less than the attractiveforce due to the potential between the roller 32 and electrode 33 thenthe toner will be stripped from its carried and deposited upon theroller 32. Positive charged particles will not develop on the roller 32since it is at a higher potential than the electrode 33. Any positivetoner which is attracted to the electrode 33 will be physically removedtherefrom by passing developer.

Assuming an image of a positive charge, when it is desired to withdrawparticles charged to the wrong polarity (positive), the roller 32 has anegative voltage V, applied thereto and the development electrode 33 haseither a positive or less negative voltage V applied thereto. Thedifference in the voltage potentials V and V is usually such as toproduce a field which will attract the positive charge particles to theroller 32 as the roller 32 rotates. The negative charged particles willnot develop on the roll 32 since the roll 32 is at the lower potentialcompared to the electrode 33. Any negative toner which is attracted tothe electrode 33 will be physically removed by and/or attracted to thepassing developer.

AS the roller 32 rotates, the scraper 36 engages the roller 32 to removeany toner particles deposited thereon. The toner particles fall bygravity over the scraper guide 36 back into the developer sump withoutengaging the photosensitive surface 12 wherein the toner particles willbe mixed with the carrier particles to become triboelectricallycorrectly charged and redistributed by the bucket conveyor over thechute 26 and past the roller 32 and onto the photosensitive surface todevelop the latent image thereon.

The connections between the motors and their associated members to bedriven thereby have been omitted for clarity and it should be understoodthat such connections can be any type wellknown in the art. The latentimage is imposed on the photosensitive surface 12 and the developedlatent image is transferred from the photosensitive surface to a copypaper by wellknown xerographic techniques as for instance described inUS. Pat. No. 3,336,905.

Referring to FIG. 2, the concept of this invention may be employed withmagnetic brush development systems. All of the elements which are thesame as those in the embodiment of FIGS. l-3 are illustrated with thesame reference number only with a small a" affixed thereto. In thisembodiment, a developer mix 100 comprises ferromagnetic carrierparticles and toner particles located in a developer sump. A magneticroller 102 is rotatably mounted in the sump and has a portion thereofdisposed within the developer mix 100 to attract developer thereto topresent the same to the latent image on the photosensitive surface 12a.A motor (shown in block form) 104 is utilized to rotate the magneticroller 102. The toner pickoff roller 32a is located adjacent themagnetic roll 102. The roller 32a has a voltage potential V appliedthereto and the magnetic brush 102 has a voltage potential V appliedthereto. The potential V may be more positive or more negative than thepotential on roller 32a depending upon whether it is desired to developon the roller 32a the weakly charged particles or the particles chargedto the wrong polarity, respectively. This is assuming a positivelycharged image. As the magnetic roll 102 rotates developer past thepickoff roll 32a, the roll 32a will attract those particles which arecharged either in the wrong polarity or the weakly charged particlesdepending upon the field produced by the potentials V and V Theparticles attracted by the roller 32a are scraped off the roller by thescraper and guide 36a and are returned back to the developer sump forre-charging and re-use.

While there is illustrated a single magnetic roll, multiple magneticrolls may be provided. Several magnetic rolls may be provided totransport developer to a developing roll or all of the rolls may bedeveloping rolls. In the former system the pickoff roll 32a may belocated adjacent the transport rolls or the first development roll topick off toner from the developer prior to the developer being presentedto the latent image. In the latter system, the pickoff roll 32a may belocated adjacent the first development roll to pick off toner from thedeveloper prior to the developer being presented to the latent image.

What is claimed is:

1. In an electrostatic processor having a photosensitive surface forcarrying an electrostatic latent image, and a development systemincluding means for transporting developer along a predetermined pathleading toward and away from said surface to develop said image; saiddeveloper containing properly and improperly charged toner particles,with the properly charged particles being triboelectrically attracted tocarrier particles; the improvement comprising the combination ofelectrically conductive means adjacent said path and upstream of saidsurface, and means for biasing said conductive means to create anelectric field which selectively attracts at least some of theimproperly charged toner particles toward said conductive means whilerepelling the properly charged toner particles therefrom, wherebyimproperly charged toner particles are removed from said developer priorto developing said image to thereby suppress background development.

2. The processor of claim 1 wherein said means for transporting saiddeveloper to said photosensitive surface comprises a chute located abovethe photosensitive surface and means for cascading said developer ontosaid chute and across said photosensitive surface, said conductive meansbeing located within the path of the cascading developer.

3. The processor of claim 1 wherein said means for transporting saiddeveloper to said electrostatic latent image comprises a magneticroller, said conductive means including said magnetic roller and anothermember adjacent said magnetic roller, with said field being establishedbetween said member and said magnetic roller.

4. The structure as recited in claim 3 wherein said other memberincludes a second magnetic roller, and the field between said magneticrollers is selected to remove toner particles charged to the samepolarity as said latent image.

5. The structure as recited in claim 1 wherein said field is selected toremove toner particles charged to the same polarity as said latent imagefrom said developer.

6. The structure as recited in claim 5 wherein said conductive meansincludes a roller on one side of said path and an electrode adjacentsaid roller on the opposite side of said path, and said biasing meansbiases said roller and said electrode to establish said fieldtherebetween.

7. In an electrostatic processor having a photosensitive surface forcarrying an electrostatic latent image of a predetermined polarity, adevelopment system for developing said latent image through the use of adeveloper comprising toner particles triboelectrically attracted tocarrier particles; the combination of means for presenting saiddeveloper to said surface to develop said electrostatic latent image,and means for subjecting said developer to an electrical field prior todeveloping said image, said field being selected to remove from saiddeveloper relatively weakly charged toner particles having a polarityopposite the image polarity, thereby reducing the quantity of tonerparticles which tend to be attracted to background areas of said imageby residual electrostatic charge onsaid surface.

of said field is a biased roller toward which said weakly charged tonerparticles are attracted.

11. The structure as recited in claim 10 wherein another terminus ofsaid field is a biased magnetic roll, and said means for presentingdeveloper to said surface includes said magnetic roll.

12. The structure as recited in claim 10 wherein another terminus ofsaid field is a biased electrode. said roller and said electrode beingpositioned so that said developer passes therebetween.

1. In an electrostatic processor having a photosensitive surface forcarrying an electrostatic latent image, and a development systemincluding means for transporting developer along a predetermined pathleading towaRd and away from said surface to develop said image; saiddeveloper containing properly and improperly charged toner particles,with the properly charged particles being triboelectrically attracted tocarrier particles; the improvement comprising the combination ofelectrically conductive means adjacent said path and upstream of saidsurface, and means for biasing said conductive means to create anelectric field which selectively attracts at least some of theimproperly charged toner particles toward said conductive means whilerepelling the properly charged toner particles therefrom, wherebyimproperly charged toner particles are removed from said developer priorto developing said image to thereby suppress background development. 2.The processor of claim 1 wherein said means for transporting saiddeveloper to said photosensitive surface comprises a chute located abovethe photosensitive surface and means for cascading said developer ontosaid chute and across said photosensitive surface, said conductive meansbeing located within the path of the cascading developer.
 3. Theprocessor of claim 1 wherein said means for transporting said developerto said electrostatic latent image comprises a magnetic roller, saidconductive means including said magnetic roller and another memberadjacent said magnetic roller, with said field being established betweensaid member and said magnetic roller.
 4. The structure as recited inclaim 3 wherein said other member includes a second magnetic roller, andthe field between said magnetic rollers is selected to remove tonerparticles charged to the same polarity as said latent image.
 5. Thestructure as recited in claim 1 wherein said field is selected to removetoner particles charged to the same polarity as said latent image fromsaid developer.
 6. The structure as recited in claim 5 wherein saidconductive means includes a roller on one side of said path and anelectrode adjacent said roller on the opposite side of said path, andsaid biasing means biases said roller and said electrode to establishsaid field therebetween.
 7. In an electrostatic processor having aphotosensitive surface for carrying an electrostatic latent image of apredetermined polarity, a development system for developing said latentimage through the use of a developer comprising toner particlestriboelectrically attracted to carrier particles; the combination ofmeans for presenting said developer to said surface to develop saidelectrostatic latent image, and means for subjecting said developer toan electrical field prior to developing said image, said field beingselected to remove from said developer relatively weakly charged tonerparticles having a polarity opposite the image polarity, therebyreducing the quantity of toner particles which tend to be attracted tobackground areas of said image by residual electrostatic charge on saidsurface.
 8. The processor of claim 7 wherein said means for presentingsaid developer to said surface comprises a chute located above thephotosensitive surface and means for cascading said developer along apredetermined path onto said chute and across said photosensitivesurface, said field traversing said path.
 9. The processor of claim 7wherein said means for presenting said developer to said surfacecomprises a magnetic roller, and said field is established between saidmagnetic roller and a member located contiguous thereto.
 10. Theprocessor of claim 7 wherein one terminus of said field is a biasedroller toward which said weakly charged toner particles are attracted.11. The structure as recited in claim 10 wherein another terminus ofsaid field is a biased magnetic roll, and said means for presentingdeveloper to said surface includes said magnetic roll.
 12. The structureas recited in claim 10 wherein another terminus of said field is abiased electrode, said roller and said electrode being positioned sothat said developer passes therebetween.